Methods of producing semiconductive bodies



Dec. 20, 1955 G. K. TEAL 2,72 7,840

METHODS OF PRODUCING SEMICONDUCTIVE BODIES Filed June 15, 1950 2 Sheets-Sheet l 34 I sa 1 i FIG. 3 P

INVENTOR G. K. TEAL BVaM A 7' TO/PNEY 1955 G. K. TEAL 2,727,840

METHODS OF PRODUCING SEMICONDUCTIVE BODIES Filed June 15, 1950 2 Sheets-Sheet 2 Ll mfimflifh #2 I i Q W W.

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N k' k INVENTOR GKTEAL ATTORNEY United States Patent Ofifice 2,727,840 Patented Dec. 29, 12955 METHODS OF PRODUCING SEMICONDUCTIVE BODIES Gordon K. Teal, Summit, N. 1., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 15, 1950, Serial No. 168,184

8 Claims. (Cl. 148-15) The invention relates to methods of producing semiconductive bodies and more specifically to methods, of the general type disclosed in the application Serial No. 138,354, filed January 13, 1950, now Patent 2,683,676, granted July 13, 1954, of J. B. Little and G. K. Teal, for producing crystals of germanium.

Semiconductive bodies, and particularly those of germanium and silicon, find application in a variety of signal translating devices, such as rectifiers, detectors, modulators and amplifiers. In one type of such devices, of which those disclosed in the application Serial No. 35,423, filed June 26, 1948, now Patent 2,569,347, granted September 25, 1951, of W. Shockley and Patent No. 2,502,479, granted April 4, 1950, to G. L. Pearson and W. Shockley are illustrative, the semiconductive body includes two or more contiguous regions or zones of opposite conductivity types.

The operating characteristics of devices including zones of opposite conductivity types are dependent markedly upon the physical and electrical character of the crystalline material. For example, resistances in such devices are dependent upon the impurity content of the material. Also for example, in semiconductor amplifiers, the efficiency and gain are dependent upon the lifetimes of the charge carriers, viz, electrons in P-conductivity type material and holes in N-conductivity type material. In general, long carrier lifetimes are eminently advantageous.

One general object of this invention is to facilitate and to improve the production of semiconductive bodies and particularly of single crystals of germanium especial- 1y suitable for use in signal translating devices.

More specific objects of this invention are to improve the characteristics of such crystals whereby long charge carrier lifetimes are obtained and to enable the production of homogeneous bodies of germanium and silicon, contiguous zones of which have different conductivities or conductivity types, and to enable control or gradation of the conductivity in one or more of such zones or regions in proximity to a P-N junction.

In one illustrative embodiment of this invention, a single crystal of germanium is produced by partially immersing a seed of germanium in a molten mass of germanium material and withdrawing the seed under controlled conditions and at a rate such as to draw some of the mass along with the seed, whereby an elongated crystal or rod of germanium is obtained. The seed and the molten mass may be of the same or opposite conductivity type.

In accordance with one feature of this invention, during the drawing of the elongated crystal or rod, the conductivity or conductivity type of the molten mass is selectively altered in a prescribed manner one or more times, thereby to correspondingly control the characteristics of successive regions or zones of the resulting crystal. For example, in one specific and illustrative case wherein the melt is initially of N-conductivity type germanium material, after a portion of the crystal has been drawn, this portion being of N-conductivity type,

an acceptor impurity, such as gallium, is added to the melt in quantities sufiicient to change the melt to P type. Hence, the next drawn portion of the crystal is of P type. Later during the drawing of the crystal, a donor impurity, such as antimony or arsenic, is added to the melt to change it to N type whereby the neXt succeeding portion of the drawn crystal is of this type. Thus, successive contiguous zones or regions of the final crystal are of opposite conductivity types.

Of course, the melt may be initially of P-conductivity type and altered during the growing of the crystal first to N type by addition of a donor impurity and then again to P type by addition of an acceptor impurity. Furthermore, the quantity of the donor or acceptor material added to the melt need not be sufficient to alter the conductivity type of the melt. For example, the quantity may be controlled to alter the conductivity of the resulting crystal as for instance to taper the resistance of either the N or P zone or both on opposite sides of the junction toward or away from this junction.

The invention may be practiced in methods of the general type disclosed in the Little-Teal application identified hereinabove, or in methods such as disclosed in the application Serial No. 138,354, filed January 13, 1950, of M. Sparks wherein a circulated melt is utilized.

The invention and the above-noted and other features thereof will be understood more clearly and fully from the following detailed description with reference to the accompanying drawing in which:

Fig. l is an elevational view of apparatus which may be utilized in the practice of the methods of this invention, a portion of the apparatus being broken away and parts being shown in section to illustrate details more clearly;

Fig. 2 is an exploded perspective view of the impurityintroducing mechanism included in the apparatus shown in Fig. 1;

Fig. 3 illustrates a typical crystal of semiconductive material produced in accordance with this invention; and

Fig. 4 is a diagram of another form of apparatus which may be utilized in the practice of this invention.

The invention will be described hereinafter with particular reference to the production of single crystals of germanium by a method involving the use of antimony as a donor impurity and gallium as the acceptor impurity. It is to be understood, however, that the invention may be used also in the production of silicon single crystals and that other donor and acceptor impurities may be utilized. Among the donor impurities which may be used with either silicon or germanium are phosphorus and arsenic as well as intimony and other acceptor impurities which may be utilized are boron, aluminium and indium as well as gallium. The initial material utilized may be silicon or germanium of either conductivity type. Germanium of either conductivity type may be produced in one way as disclosed in the application Serial No. 638,351, filed December 29, 1945, now Patent 2,602,211, granted July 8, 1952, of J. H. Scaif and H. C. Theuerer. Silicon material of either conductivity type may be produced in one way as disclosed in the application Serial No. 793,744, filed December 24, 1947, now Patent 2,567,970, granted September 18, 1951, of J. H. ScafE and H. C. Theuerer.

Referring to the drawing, the apparatus illustrated in Fig. 1 comprises a base 10 having mounted thereon a bell jar 11 provided with inlet and outlet ports 12 and 13 respectively by way of which a suitable gas such as hydrogen or helium may be circulated through the bell jar 11. Extending inwardly from the base 10 is a standard 14 upon which a crucible 15, advantageously of carbon for a germanium melt, is mounted. Quartz crucibles may be used for silicon melts. The crucible has therein a charge '16 of semiconductive material, specifically gerliquefied by induction heating through the agency of a coil 17 energized from a high frequency source 18.

Opposite the free surface of the charge or mass 16 is a seed 19 which is carried by a stem 20 extending'from a weight 21.- The latter is slidable longitudinally in a guide -22 and is suspended by a wire 23 which passes over pulleys 24 and is -connected to a platform 25. The platform 25 is mounted threadably upon'a drive shaft 26 rotatable by a motor 27. Upon operation of the motor 27 to rotate the shaft 26, the platform 25 is raised or lowered whereby the weight 21 and the seed '19 carried thereby are lowered or-raised'respectively.

Extending into the bell jar 11 is a pipe having at its inner end a circular portion -28 provided with a multiplicity of apertures through which jets of an appropriate gas may be played upon the materialdrawn in the man ner described hereinafter. The gas maybe, for example hydrogen, introduced into the pipe from a source, not shown,and thehydrogen may be caused to contain some water vapor from a reservoir 29 of distilled water coupled tothepipe-througha valve system 39.

The apparatus thus far described is generally thesame and is operated in the same manner as that-disclosed in the application of LB. Little and G.-K-; Teal identifie hereinabove. In addition the apparatus shown in Fig. 1 comprises 'a mechanism shown in greater detail in Fig. 2 for selectively introducing significant impurities into-the charge within the crucible 15.- This mechanism comprisesa first plate 32 having a-single aperture therein which is in communication with a spigot-33 extending over the crucible 15 A second plate 34 slidable inface-to face relation upon the --plate--32-,is rockable by a '-lever37 and has therein a pair of spaced recesses 35 in each of which a pellet 36a or 36b-is positioned; The pellets 36 .:advantageously are alloys of germanium and-a significant impurity, the impurity-in one-pellet-being-a donor-and that in the other being -an acceptor. -Either recess 35 may be aligned with the inlet and the spigot 33 by rocking of theplate 34 wherebythe pellet' contained 'in that recess passes into the spigot-33 andthence intothe charge in the'crucible 15.

Disposed in the crucible 15 is a stirrer which may be actuated by-mauipulationof the rod 51. V In the drawingof the -crystal, the charge 16 within the crucible I5 is heatedthrough the coil-1 7 and maintained in the molten state.- The motor- 27 is operated to .partly'immerse the seed 19-into the molten mass and then is operated-so that theseed is withdrawn from the massat a rate to drawsome-ofthe-molten-'=material along therewithm-As set-forth in the Little-Teahapplication above identi. fied,-the rate of withrlrawal'of the seed 19 is made substantially that'at which the -molten-materi'al Q" crystallizes.

heconductivitytype of the molten material withdrawn from the crucible will be-depen'dent, of course, upon the donor-acceptor impurity balance in the material. For'ergamp'le, if in the initial-charge the donor-impurity is in etfectiye excess, the portion-of the'mass withdrawn willgbe of' N-conductivity type. At a prescribed point in -the-withdrawingoperation, the plate 34' -may be manipulated wherebythepellet 36 containing the acceptor impurity is introduced into the melt. The amount of acceptor impurity is-rnade sufficient to change 'the molten mass to the opposite conductivity type; 'I'hus,as" tl:le withdrawal ofthe seed continues, the nextzportion of'the crystal formed-is of l -conductivity type and this portion forms a junction with the first' drawn portion. If subsequently the plate 34 is manipulated to introduce into the meltthe pellet 36 containing thei'donor imputity, the mass within thecrucible' willbewhanged fronr'P to 'N' conductivity type; The subsequently withdrawn material also will be of N-conductivity type'. Thusfb'y 4 selective introduction of impurities into "the melt during the withdrawal of the seed, contiguous zones of opposite conductivity types are produced iri the withdrawn material and in the final crystal.

A typical crystal produced in accordance with this invention is illustrated in Fig. 3 and as there shown comprises the seed 19 followed' in succession by zones of N-, P- and N conductivity type respectively.

'It will beunders'tood, of course, that "the charge 'or mass 16 may be initially P-conductivity type and by introduction of appropriate impurities changed first to N- conductivity type and then again to P-condu'ctivity type. It will be understood further that any desired number of changes in conductivity type of the melt may be effected during'the production of a single crystal.

In one specific example illustrative of the invention a germanium body comprising an N-P junction was produced in the following manner: A high purity N type single crystal of germaniumwas grown in the apparatus illustrated in Fig. -1 in the general manner disclosed in the application of l. '13. Little -and*G. Teal identified hereinabove. The initial high purity material was produced by reduction of germanium dioxide inhydrogen in the manner disclosed in the application of I. H.'Scatf and H. C. Theuerer-also --identified-hereinabove. During the growth of the N type crystal, the composition of the germanium melt was-altered by dropping thereinto a pellet of an alloy-of germanium and gallium. The melt was thereby converted from N-conductivity type to P type, whereby the drawn crystal was a rod shaped germanium single crystal including a transverse P-N junc-' tion. The initial germanium'material wassubstantially 50 grams, the P-N junction was located'about one-half way down thelength-of the drawn crystal, and the added alloy was a 20 milligram pellet of 0.2 per cent gallium ingermanium.

Thetotals-hydrogen fiow in the apparatus was approximately 100 cubic feet per :hourwith about-Z-cubicfeetper hour fromthejets around the growing crystal.

--In a typical element including a -P-N junction-prepared in this :manner the resistivity was '6 ohm-centimeters on the N typeside of the-junction and 0.6-ohmcentimeter on the P type side of the junction. The carrier lifetimes -on the two sides of the junction were of the order-of to microseconds. These values are substantiallyequal to those determined for the junctionregion.

* As another example, a germanium element having a P type zone-between wand contiguous with-two N type zones 'was fabricated-in the :following maunerz First an Nsection-orzonvwas grown from an N type'melt and :1 V pellet of germanium alloy of -l gallium -was dropped into the :mlt during the growth of the crystal, thereby to produce 'an -N-P junction and a P section or zone in the drawn --crystal.-- Then, specifically a'few-seconds after introduction Ofl-ihfi first .pellet, a second :pellet of an antimony alloy of germaniumwas 'dropped into the melt--thereby converting the melt back to :N type, whereby an N -typesection was crystallized; -The resultant crystal. was -a-single crystal-of germanium comprising a thin P type layer-or zone, of--the order of 30 mils thick, between two regions or zones of N type germanium.

1 The-environmental conditions during the growth of the PNP"'structure were substantially the i's'ame as those set forth-hereinabove in the discussion ofthe exemplary P-N structure. The initial germanium was an-ingot of high ipurity --germanium,--the--first pellet-"of germanium alloy was essentially the same as that described hereinabove-and the second pellet -was;a' I5 milligram pellet of 6.4 per'eentantimony inge'rmanium.

--The:resulting'ctructure was-operated "as a transistor of theform disclosed in the application oLW. Shockley hereinabove identified ;and="eirhibited long *cari'ier lifetimes in -the iransiiicn l type region or zone, exceptionally low 'noise 'andhigh powergains.

"when The apparatus illustrated in Fig. 4 comprises a trough 40"along which molten semiconductive material, germanium or silicon, is flowed from a reservoir 41 heated as by an induction coil 42, the reservoir being connected to the trough by way of a valve 43. The molten material may be discharged to a second reservoir 44 connected to the trough by way of a valve 45. Circulation may be effected through the agency of a suitable pump 46. The material in the second reservoir 44 may be purified by passing it through a suitable purification chamber, not shown, connected to the reservoir 44 by ports, one of which is shown at 47.

A seed 190 is partially immersed in the molten material within the trough 40 and is withdrawn by appropriate mechanism not shown in the same manner as the seed 19 in the embodiment of the invention heretofore described to withdraw some of the molten material therewith. Insertable into the molten material are rods 48a and 48b of an alloy of germanium and a significant impurity. For example, the rod 48a may contain a donor impurity and the rod 48b an acceptor impurity.

As the seed 190 is withdrawn from the molten material flowing in the trough 40, the conductivity type of the molten mass may be changed by inserting one or the other of the rods 48 into the mass. Thus, if the mass is initially of N-conductivity type, the acceptor impurity-containing rod is immersed into the mass thereby to change it to P-conductivity type. The P-conductivity type mass may be changed to N-conductivity type by insertion thereinto of the donor impurity-containing rod 48. Thus, it will be seen that as in the method hereinbefore described with reference to Fig. l, the conductivity type of successive regions of the material withdrawn from the melt by upward motion of the seed 190 may be altered so that NPN or PNP type single crystals are produced.

Although the invention has been described thus far with particular reference to the alteration of the conductivity type of the molten mass and, hence, of the resulting single crystal, it will be appreciated that by appropriate control of the amount of donor or acceptor impurities introduced into the molten mass, a selective and prescribed change in the conductivity of the material and, hence of successive regions or zones of the final crystal may be realized.

Although the invention has been described with particular reference to introduction of the significant impurity in the solid state, such impurity may be introduced into the melt in other ways, for example in the form of a gas directed against the melt as through a jet, not shown, in proximity to the surface of the melt. Illustrative of the gases which may be employed are boron hydride and antimony hydride.

It will be appreciated also that although specific embodiments of the invention have been shown and described, they are but illustrative and that various modifications may be made therein without departing from the scope and spirit of this invention.

What is claimed is:

1. A method of making a semiconductive element which comprises melting a mass consisting essentially of a predominating conductivity-type determining impurity and semiconductive material selected from the group consisting of germanium and silicon, introducing into a molten mass a seed consisting essentially of the same semiconductive material as in'the mass and a conductivity-type determining impurity, withdrawing the seed at a rate to withdraw some of said molten mass along therewith, then without separating the seed and withdrawn material completely from the molten mass, adding a conductivity-type determining impurity to the molten mass to alter the conductivity type of the conductivitytype determining impurity predominating in the molten mass, further withdrawing said seed away from the molten mass at a rate to withdraw an additional portion of the molten mass, and allowing the withdrawn material to crystallize. v

'2. The method of making a semiconductive element which comprises melting a mass consisting essentially of a predominating conductivity-type determining impurity and semiconductive material selected from the group consisting of germanium and silicon, introducing into the molten mass a seed consisting essentially of the same semiconductive material as in the mass and a conductivity-type determining impurity, withdrawing the seed at a rate to withdraw some of said mass along therewith, then without removing the seed and withdrawn material completely from the mass adding a conductivitytype determining impurity to the molten mass to control the electrical properties of the material withdrawn, fur ther drawing said seed away from the molten mass at a rate to draw an additional portion of the mass, and allowing the Withdrawn material to crystallize.

3. The method of making a semiconductive element which comprises melting a mass consisting essentially of semiconductive material selected from the group consisting of silicon and germanium and a conductivity-type determining impurity such that the material withdrawn from the melt will after solidification be of one conductivity type, introducing into the molten mass a seed consisting essentially of the semiconductive material of the mass and a predominating conductivity-type determining impurity characteristic of the one conductivity type, withdrawing the seed at a rate to draw some of said mass along therewith and to allow the material withdrawn to solidify, then without separating the seed and the withdrawn material completely from the mass adding a conductivity-type determining impurity to the molten mass to alter the conductivity type after solidification of the withdrawn material, and further with drawing said seed at a rate to draw an additional portion of the mass which is allowed to solidify.

4. The method of making a semiconductive element which comprises melting a mass consisting essentially of semiconductive material selected from the group consisting of silicon and germanium and a conductivity-type determining impurity such that the material withdrawn from the melt will after solidification be of one conductivity type, introducing into the molten mass a seed consisting essentially of the same semiconductive material as in the mass and a predominating conductivity-type determining impurity characteristic of the one conductivity type, withdrawing the seed at a rate to draw some of said mass along therewith and allowing the material withdrawn to solidify, then without separating the seed and the withdrawn material completely from the mass adding a conductivity-type determining impurity to the molten mass to control the electrical properties after solidification of the withdrawn material, and further withdrawing said seed at a rate to draw an additional portion of the mass which is allowed to solidify.

5. The method of making a germanium element which comprises melting a mass consisting essentially of germanium and a predominating conductivity-type determining impurity, introducing into the molten mass a seed consisting essentially of germanium and a conductivitytype determining impurity of the same conductivity type as that in the molten mass, withdrawing said seed away from said mass to uplift material which is allowed to solidify, and then without separating the seed and the uplifted material completely from the mass adding a conductivity-type determining impurity to control the concentration of conductivity-type determining impurities in the mass.

6. The method of making a germanium element which comprises melting a mass consisting essentially of germanium and a predominating conductivity-type determining impurity, introducing into the molten mass a seed consisting essentially of germanium and a conductivitytype determining impurity of the same conductivity type alter thecdn ltidti'fity iyp'eiiftei solidification o' fthe matefia l lieing Withdrawn.

" l/The method "of makinga 'se'rnicondlictive crystal Having flierein "contiguous zones of opposite conductivity type whih'compfises melting amass"'ofisemiconductive mate ialseleeredfrem*thegroupconsistin of germanium and "silicon and awredomimnt conductivity-type determining ixnpiirity,introducing into the molten mass a seed c'o'nsistirig' ess'e'xitiall'yfif the same "semiconductive material as me 'fma'ss and'a "conductivity type' determining ind purityoftlie "same conductivity type as that-in the mass. Withdi'a'wing' saidseedata rate to'with'draw some ofthc mass along therewith and to allow/"the 'material'withdrawn tosblidify, and at spa'ced times in the period of Withdrawal adding fitsftdthe 'molten'mass 'a' conductivitytype "detefinining'impurity (if-type and quantity so that rheconduct i fly type 'aftei solidificati'on of the material being'withdrawnis -of op'pos'it'e conductivity type and snbseqnently a conductivity 'typ'e'determining "impurity Uftyp'e and quantity so that the-conductivity type after vsolidl'fiea'tioxr'of the "material being withdrawn is 0": the oi-lginal 'conduc'tiv'itytype.

8. In the production of a monocrystalline element eensistin' essentiall of ge manium and r'a *eonduetivit type determinin impurity, by withfl'raw'i g'a seed from a melten mass "consisting essentially ofigermanium "and conduc'tiiitfitylie" determining impurity at a rate to drawsome o'flhe "mass; "aldng' wilh-eefid-"seeh and to allow the withdrawn material to'crystallize trie method Whih comprises in'tf'oduciing alternatelyduring' th period of Withdrawal donor and "acceptor impurities'into thenrdlten mas'sfio alternate the Conductivity type after crystallization (if successive regions -of the withdfay' vn material.

References Cited in the file ofthis patent UNITED STATESTPATENTS 

1. A METHOD OF MAKING A SEMICONDUCTIVE ELEMENT WHICH COMPRISES MELTING A MASS CONSISTING ESSENTIALLY OF A PREDOMINATING CONDUCTIVITY-TYPE DETERMINING IMPURITY AND SEMICONDUCTIVE MATERIAL SELECTED FROM THE GROUP CONSISTING OF GERMANIUM AND SILICON, INTRODUCING INTO A MOLTEN MASS A SEED CONSISTING ESSENTIALLY OF THE SAME SEMICONDUCTIVE MATERIAL AS IN THE MASS AND A CONDUCTIVITY-TYPE DETERMINING IMPURITY, WITHDRAWING THE SEED AT A RATE TO WITHDRAW SOME OF SAID MOLTEN MASS ALONG THEREWITH, THEN WITHOUT SEPARATING THE SEED AND WITHDRAWN MATERIAL COMPLETELY FROM THE MOLTEN MASS, ADDING A CONDUCTIVITY-TYPE DETERMINING IMPURITY TO THE MOLTEN MASS TO ALTER THE CONDUCTIVITY TYPE OF THE CONDUCTIVITYTYPE DETERMINING IMPURITY PREDOMINATING IN THE MOLTEN MASS, FURTHER WITHDRAWING SAID SEED AWAY FROM THE MOLTEN MASS AT A RATE TO WITHDRAW AN ADDITIONAL PORTION OF THE MOLTEN MASS, AND ALLOWING THE WITHDRAWN MATERIAL TO CRYSTALLIZE. 